The invention relates to a novel process for treatment of a combination of gases, finely divided solids, and liquids using a wire filter element with mechanically controlled filter slit of precise minute openings in the range of 0.5 to 100 microns. The process is used where reactions or other functions are carried out with finely divided solids in intimate contract with liquid or gases or both. The filter element will retain solids larger than the slit openings of the element. As an example of the application of the filter element with the process, the filter slit openings are 10 microns and the solid particle sizes are greater than 20 microns in the reactor vessel and only the liquid product is withdrawn through the filter slits while all the solid particles are retained in the vessel.
A number of established and developing chemical, petroleum refining and other continuous processes use finely-divided catalysts to promote reactions in vessels typically operated at elevated temperatures and pressures. Economics strongly favor the ability to use the catalyst for prolonged periods without shutting down the process or without losing or replacing the catalyst. Some of these processes retain the catalyst in the reactor, or control the loss to a small mount each day, by the use of cyclones, filters, electrostatic precipitators, scrubbers, or combinations of these and other techniques. Successful techniques have been proven for commercial use for gas-solid catalyst systems such as fluidized-bed catalytic cracking. No such technique has been developed for either of the analogous liquid-solid or the gas-liquid-solid systems.
An example of an unfulfilled need is the rapidly-increasing use of Slurry Bubble column Reactors (SBCR) in which a means for catalyst retention within the reactor remains an unsolved problem. This was the conclusion in a paper by B. Bhatt, et al., titled "Liquid-Phase Fischer-Tropsch Synthesis in a Bubble column," published by U.S. Department of Energy Pittsburgh Technology Center in a compilation "Liquification Contractor's Review Conference" Proceedings, Sep. 22-24, 1992. As stated in the paper "No single proven technology exists in the public domain. . . " (to retain the catalyst). The size range selected for catalyst in SBCR's depends on many factors. Some of these are: (a) manufacturing capability; (b) catalyst performance; (c) suspension requirements; and (d) reactor design and hydrodynamics. Therefore, no matter which size range is chosen, there is a need for a special device to retain the catalyst within the reactor. The liquid product must be able to flow through this device, while the gaseous and solid phases are retained in the reactor.
The B. Bhatt. et al., paper cited above reported significant daily losses of a finely-divided iron-based catalyst, despite best efforts to recover the catalyst. Another similar operation using an iron-based catalyst also led to large catalyst losses. In tests and many operations using fine particles, significant mounts are smaller then 10 microns particle diameter. The tendency of porous metal, fine-wire matts and other designs to rapidly and often permanently plug or "blind" has been encountered. These typically are based on filtering through a maze of interstices which usually are variable in size. Thus fine particles which enter the maze tend to become trapped inside the porous structure, leading to plugging of many of the passageways. Another buildup often occurs at the surface of most filters, where the deposited solids compact into a cake and progressively increase the pressure drop. A reverse surge of fluid called blowback will often reduce the cake thickness or even remove nearly all of it. However, blowback will rarely remove the majority of the particles from within the filter medium interstices. This eventually plugs the filters. Another advantage of the shaped-wire filter is the higher capacity for a given area of openings when compared to the porous-type filters. These latter, having a relatively-long tortuous pathway through the pores, create more pressure drop than the shaped-wire with a very short restricted path.
Solids loss and filter plugging are both accentuated when the solids contain a large amount of very fine materials. These fine materials may result from improper preparation of the solids to be used, or may be generated by attrition in the churning bed of solids. Attrition is minimized by (1) careful design of the bed so that spots where high velocity mixing may occur are eliminated, and (2) by use of solid particles which are hard, strong and with a smooth surface which translates to "attrition resistant." Fine materials found in the freshly prepared solids can be removed before charging to a reactor or within a reactor using a controlled stream of gas or liquid to lift or elutriate and thus remove a specific size range. Other methods such as hindered settling or classification are useful for this purpose in some cases.
Slurry Bubble Column Reactor use to produce liquid fuels from a hydrogen-carbon monoxide mixture has recently drawn a great deal of attention from a number of research facilities. Much of the work in the United States has been coordinated and supported by the U.S. Department of Energy as part of a long-range policy to increase availability of liquid fuels. Some privately-funded research and development has also been carried out, over a span of a decade or more. In each effort the retention of catalyst in the reactor has been found to be difficult or unachievable. Thus the processes are not commercially feasible until this problem is solved.
Fixed-bed Fischer-Tropsch reactors were used for liquid fuel production in Germany during World War II. Research and development has continued in several countries since then. South Africa has used their fixed=bed reactors for commercial production for more than two decades. A commercial plant has been built by Royal Dutch Shell Company in Malaysia. These large plants have established the investment and operating costs for the fixed-bed processes. Developments and preliminary engineering designs for large Bubble Column Reactor use for the Fischer-Tropsch reactions indicate that lower investment and operating costs with highly desirable products should result. Another important advantage of the Slurry Bubble Column Reactor is the greatly-improved heat removal capability and ease of temperature control when compared to the fixed-bed Fischer-Tropsch reactors.
In addition to the above advantages are the possibilities of drawing off increments of aged catalyst during long operations and of adding increments of fresh or regenerated catalyst to the slurry reactor. These are not possible during operation of a fixed-bed Fischer-Tropsch reactor.
Various attempts have been unsuccessfully made to limit filter slit widths to a precise minute dimension of about 20 microns. Korchi Arai in his U.S. Pat. No. 5,047,148 for "Retained Wire Filter Element" represents the forming of filter slits with an accuracy of one micron to one millimeter by anchoring a specially shaped wire into matching grooves in a plate. The distance between the grooves in the plate, minus the width of the top surface of the specially shaped wire would determine the filtering slit width with the accuracy claimed. Since the filtering slit width in the Arai technique is subject to the sum of at least two machining tolerances, it appears unlikely that a slit width accuracy represented by Arai can be attained.
Fritted metal and fine-wire screens have been made with openings averaging as small as a few microns. However, it has been found that such filters display wide variances in the size of the space inside the relatively deep structure of the filter medium. The result is that fine particles infiltrate and are trapped inside the structure resulting in plugging and blinding.
This invention overcomes the limitations of heretofore known processes which did not have the capability of retaining catalysts or solids of fine sizes. The process of this invention uses filter elements having filtering slits of precise minute widths in the range of 0.5 to 100 microns whereby catalysts or solids of fine sizes are retained and at the same time achieving the filtering advantages of shaped wire filtering elements. Such filters and their use in the process of this invention would enable long operating periods such as a year or more, with catalyst losses confined largely to the fines generated by attrition. The process could use a single filter or double filters with blowback provision for insurance against temporary plugging from process upsets.